# Origins of collectivity in small systems

## TL;DR

This paper reviews recent theoretical advances in understanding multi-particle correlations in small collision systems, exploring whether initial or final state effects dominate the observed collective phenomena.

## Contribution

It highlights the potential roles of initial state momentum correlations and final state interactions in small systems, emphasizing the importance of proton shape fluctuations.

## Key findings

- Initial state momentum correlations are significant in small systems.
- Results depend on proton shape and fluctuations.
- Final state interactions may influence observed correlations.

## Abstract

We review recent developments in the theoretical description and understanding of multi-particle correlation measurements in collisions of small projectiles (p/d/$^3$He) with heavy nuclei (Au, Pb) as well as proton+proton collisions. We focus on whether the physical processes responsible for the observed long range rapidity correlations and their azimuthal structure are the same in small systems as in heavy ion collisions. In the latter they are interpreted as generated by the initial spatial geometry being transformed into momentum correlations by strong final state interactions. However, explicit calculations show that also initial state momentum correlations are present and could contribute to observables in small systems. If strong final state interactions are present in small systems, recent developments show that results are sensitive to the shape of the proton and its fluctuations.